1. Field of the Invention
The present invention relates to a start control device for starting a vehicle passenger protecting device such as an air bag or a seat belt pretensioner, adapted to detect the collision of a vehicle and operate.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional air bag start control device described in Japanese Patent Publication No. 59-8574, for example. Referring to FIG. 1, reference numeral 1 denotes an acceleration sensor (G sensor) for converting an acceleration due to an impact against a vehicle into an electrical acceleration signal (which will be hereinafter referred to as an acceleration signal) and outputting the acceleration signal; 101 denotes an integrating circuit for integrating the acceleration signal from the acceleration sensor 1; 102 and 105 denote comparators for comparing the output from the integrating circuit 101 with comparative values V1 and V2, respectively; 103 denotes a time constant circuit consisting of a resistor R2, a capacitor C2, and a diode D2, for receiving the output from the comparator 102; 104 denotes a comparator for comparing the output from the time constant circuit 103 with the output from the integrating circuit 101; 106 denotes an oscillator for oscillating a reset pulse according to the output from the comparator 105; and 107 denotes a differentiating circuit consisting of a capacitor C1 and a resistor R1, for differentiating the output from the oscillator 106 and supplying an output signal as a reset signal through a diode D1 to the integrating circuit 101.
An air bag must be started according to an acceleration generating upon collision of a vehicle, for example. However, if a start signal for starting the air bag is generated, according to an acceleration signal only, the start signal would be undesirably generated in the case where a small object as hardly damaging a vehicle body attacks in the vicinity of the acceleration sensor at high speeds. In such a case, a very large acceleration with a short duration is generated. For example, such a very large acceleration is generated when an impact is applied with use of a hammer in the vicinity of the acceleration sensor. In such a case of the impact due to hammering, the start control device must not be operated. If the start control device is operated in response to hammering, there is a possibility that a very dangerous situation may occur. Thus, it is necessary to prevent that the start control device is operated against the impact causing a very large acceleration with a short duration.
To cope with this problem, the above-mentioned prior art device is designed to integrate the acceleration signal for a given time and thereby smoothen the acceleration signal. That is, the integrating circuit 101 integrates the acceleration signal from the acceleration sensor 1 to convert the acceleration signal into a velocity signal. The integrating circuit 101 is reset at fixed intervals by the reset signal from the differentiating circuit 107 that has differentiated the output signal from the oscillator 106. In other words, a velocity change is detected at fixed intervals. Then, the comparator 104 compares the output from the integrating circuit 101 with a fixed value and generates a start signal when the output exceeds the fixed value.
If the acceleration changes rapidly just before generation of the reset signal, the integrating circuit 101 would be reset during the course of the acceleration change, resulting in no reflection of this change to the output from the integrating circuit 101. That is, there is a possibility that the start signal would not be generated in spite of the situation that the start signal must be generated. To cope with this problem, the comparator 105 is provided to delay a reset timing when the acceleration changes rapidly. The oscillator 106 is of a variable frequency type.
When the output from the integrating circuit 101 exceeds the comparative value V1, the comparator 102 generates a signal to operate the time constant circuit 103. When the output from the time constant circuit 103, i.e., a predictive start level, becomes lower than the output from the integrating circuit 101 after a predetermined time, the comparator 104 generates a start signal. When the output from the integrating circuit 101 exceeds a predetermined predictive level, the output from the comparator 105 becomes low to increase the oscillation cycle of the oscillator 106. As a result, the cycle of generation of the reset pulse to be supplied through the differentiating circuit 107 to the integrating circuit 101 is increased. The predetermined predictive level mentioned above is a level where a rapid change in acceleration is predicted, and it corresponds to the comparative value V2 in the comparator 105.
The increase in generation cycle of the reset pulse results in an increase in period of detection of the velocity change. In accordance therewith, it is necessary to increase the comparative voltage in the comparator 104. Unless the comparative voltage is set higher, the start signal would be generated even upon an instant velocity change. When the acceleration changes rapidly to cause that the output from the integrating circuit 101 exceeds the comparative value V1, a high-level signal is output from the comparator 102 to result in an increase in the output voltage from the time constant circuit 103. Accordingly, the comparative voltage in the comparator 104 is increased. In this manner, even when the acceleration changes rapidly just before generation of the reset signal to the integrating circuit 101, the start signal can be accurately generated.
However, also in case of a head-on collision at medium to high speeds (e.g., about 50 km/h), the above-mentioned function becomes effective. That is, there is a problem that the increase in the comparative voltage in the comparator 104 causes delay of decision of the collision.